Most existing concrete towers, are pre-compressed (or “pre-stressed”) to account for extreme loads, such as winds that may affect the integrity of their structure. Typically these towers have a reinforced concrete column fitted with tensioning cables, such as steel cables. Towers for wind turbines may be steel, concrete or hybrid towers. Hybrid towers may have a lower concrete section and an upper steel section.
FIG. 1 shows a typical wind turbine tower. A concrete tower section has been indicated. FIG. 2 is a cross-section of the tower section of FIG. 1 with a typical arrangement of tensioning cables. The tensioning cables exert a compression on the tower section to to avoid or reduce the possibilities of the concrete section being submitted to tension under the influence of a load, such as a wind load. As the cables must account for extreme events, such as ripples of high wind, the towers are precompressed to withstand loads caused by these extreme events and the cables are accordingly tensioned.
FIG. 3 shows the negative and positive stress distribution in the base of a tower under a wind load. The point suffering the highest negative stress is point A in FIG. 3. This is the windward point at the base of the tower. A tensioning cable at point A must be pre-tensioned to counteract the negative stress caused by wind load W. The tensioning cables may generally be equally pre-tensioned around the base as wind loads may be expected from all sides. As a consequence, when a windward tensioning cable counteracts a wind load, a leeward tensioning cable simply adds compression to the leeward point (point B in FIG. 3) that is already under compression by the wind load. This means that the tower has to be dimensioned to withstand compression that is at least double the compression exerted by the tensioning cables. Consequently, the cross-section of the tower is calculated accordingly. Therefore, large amounts of concrete are required to account for this additional compression. This has a direct impact on the cost of construction of a tower.